Those skilled in the art are aware that transmissive displays such as liquid crystal displays have electro-optical characteristics which depend strongly on the angular conditions of observation. To overcome this drawback, it has been proposed to use a source of collimated light to illuminate the electro-optical modulator.
Several systems enabling collimated light to be obtained are known to those skilled in the art. Thus, the article from SID 94 Applications Digest entitled: "Flat collimator a back lighting assembly utilizing microprism for energy efficiency" describes a collimation system comprising a fluorescent tube, a substantially parabolic reflector surrounding the tube, a collimation section extending the reflector and a light duct or waveguide provided with a microprismatic structure on its lower surface. In this case, the light emitted by the tube is sent by the reflector into the collimation section and then into the light duct where, by specular reflection, on the microprismatic structure, substantially all of the light injected into the light duct is sent while being appropriately collimated towards the modulator provided parallel to the upper surface of the light duct.
A system of this type is represented diagrammatically in FIG. 1 in which reference 1 corresponds to the tube forming the light source, reference 2 to the collimation section, reference 3 to the light duct or waveguide, reference 4 to the microprismatic structure. In this case, and as represented in FIG. 1, the waveguide or light duct 3 comprises an upper face 3a on which the prismatic structure 4 is made, a lower face 3b or exit face and a rear face 3c. Furthermore, the entrance face of the collimation section is parallel to the entrance face of the waveguide. This waveguide structure has a number of defects. Thus, certain substantially horizontal light rays such as the ray r are reflected off the rear face 3c and are sent to the source. They do not serve to illuminate the modulator. This results in medium efficiency of the system. Furthermore, the prismatic structure being a homogeneous structure, namely all the microprisms being identical, a spatial inhomogeneity is observed in the direction of the exit lobe as are a number of spurious lobes due to the use of a unidirectional collimator parallel to the waveguide.